The PCB can be built either by ordering it from a fabrication house or simply DIY. But I think ordering is the best option because PCB manufacturing is really cheap these days. And also soldering SOP-16 package could be quite challenging. 

Use an IC base to mount the microcontroller in case you need to update the firmware along the way. Gerber files and the schematic can be found in the files section.

The easiest way to upload the firmware into the Attiny85 chip is by using an Arduino as the programmer. (Assuming you already have an Arduino board ling around) 

1. Open Arduino IDE and go to preferences then copy and paste the following link in the "Additional Boards Manager URLs:" 

link- https://raw.githubusercontent.com/damellis/attiny/ide-1.6.x-boards-manager/package_damellis_attiny_index.json

2. Go to Tools -> Boards -> Boards Manager.. Then search for attiny and install the package containing ATtiny85 board.

3.  Open the "ArduinoISP" sketch from the example menu and upload it to an Arduino board.

4. Connect the ATtiny85 and the Arduino board as follows.

       Arduino   ---------   ATtiny85

•      10         ---------         1
•       11         ---------         5
•      12         ---------         6
•      13         ---------         7
•     5V         ---------        8
•    GND      ---------         4

5. Open the firmware SPD.ino which can be found on this GitHub repository

6. Go to  Tools -> Programmer: and choose "Arduino as ISP"

7. Select "Sketch" in the menu bar and select "Upload Using Programmer"

You can find the 3D models for the project on Thingiverse.  All the models can be printed without supports except button ring model. I recommend print these parts with 0.1 mm layer height and 100% infill.

Screw and the long nut were designed with very low tolerance between them for reduce the backlash. So, they might need some light sanding. I used 600 Grit sand paper for this.